1. Field of the Invention
This invention relates to electroacoustic apparatus, particularly to bulk mode acoustic delay devices.
2. DESCRIPTION OF THE PRIOR ART
In the prior art a broad band delay device or delay line was made from either coiled lengths of coaxial cable or waveguides where the length of the cable or waveguide determine the time delay. The delay device could, for example, delay a radar signal for a specific period of time such as 100 nanoseconds which is normally used in conjunction with other signal processing circuitry. Delay devices made from coaxial cable or waveguides are considered to be large and heavy when used in airborne applications. An example of a broad band delay device which overcomes these objections for airborne use is described in U.S. Pat. No. 3,942,139 issued on Mar. 2, 1976 entitled "Broad Band Microwave Bulk Acoustic Delay Device" by H. W. Cooper and J. de Klerk and assigned to the assignee herein. In U.S. Pat. No. 3,942,139 a transducer generates bulk mode acoustic waves towards a second surface which reflects the waves back towards the first-mentioned transducer and a receiving transducer located nearby. The acoustic wave is reflected off the surface supporting the transducers back towards the reflecting surface whereupon the wave is reflected back to the receiving transducer. The second wave to be reflected off the reflecting surface is known as the triple transit signal or echo and is undesired. In another arrangement of transducers a quadruple transit signal occurs which is undesired. Subsequent undesired echoes are also observed but are somewhat reduced due to the effects of the attenuation in the acoustic material, for example, 3 dB per centimeter at 4 GHz for sapphire and by diffraction of the acoustic wave as it continues to spread laterally as it propagates through the transmission medium. In addition, the echoes may be partially absorbed by covering the surfaces of the delay device with a film of silver-indium to absorb unwanted unwanted acoustic waves. In certain applications, additional attenuation of the triple or quadruple transit signal is desired.
It is therefore desirable to provide additional techniques for improving the attenuation of triple or quadruple transit signal in a delay device using bulk acoustic waves and destructive interference in a low loss transmission medium such as sapphire, titanium dioxide, or lithium niobate.
Furthermore, it is desirable that additional attenuation techniques be provided for reducing the triple or quadruple transit signal which are applicable over a broad frequency range.